Alnylam Pharmaceuticals (ALNY) 10-Q 2 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	Apr. 30, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	ALNY
Entity Registrant Name	ALNYLAM PHARMACEUTICALS, INC.
Entity Central Index Key	0001178670
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		106,536,778

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 724,959	$ 420,146
Marketable debt securities	519,578	662,803
Marketable equity securities		1,206
Accounts receivable, net	33,801	18,760
Inventory	32,001	24,068
Prepaid expenses and other current assets	81,674	73,713
Total current assets	1,392,013	1,200,696
Property, plant and equipment, net	341,712	320,658
Operating lease right-of-use assets	226,412
Restricted investments	44,825	44,825
Other assets	9,037	8,623
Total assets	2,013,999	1,574,802
Current liabilities:
Accounts payable	22,671	59,708
Accrued expenses	106,873	112,719
Operating lease liability	16,977
Deferred rent		3,571
Deferred revenue	2,642	3,496
Total current liabilities	149,163	179,494
Operating lease liability, net of current portion	282,300
Deferred rent, net of current portion		57,920
Deferred revenue, net of current portion	478	458
Long-term debt	30,000	30,000
Other liabilities	5,072	4,965
Total liabilities	467,013	272,837
Commitments and contingencies (Note 14)
Stockholders’ equity:
Preferred stock, $0.01 par value per share, 5,000 shares authorized and no shares issued and outstanding at March 31, 2019 and December 31, 2018
Common stock, $0.01 par value per share, 125,000 shares authorized; 106,400 shares issued and outstanding at March 31, 2019; 101,177 shares issued and outstanding at December 31, 2018	1,064	1,011
Additional paid-in capital	4,601,662	4,175,139
Accumulated other comprehensive loss	(32,853)	(33,213)
Accumulated deficit	(3,022,887)	(2,840,972)
Total stockholders’ equity	1,546,986	1,301,965
Total liabilities and stockholders’ equity	$ 2,013,999	$ 1,574,802

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	106,400,000	101,177,000
Common stock, shares outstanding	106,400,000	101,177,000

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) shares in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues:
Revenues	$ 33,294,000	$ 21,899,000
Costs and expenses:
Cost of goods sold	3,347,000
Research and development	129,127,000	96,857,000
Selling, general and administrative	89,608,000	72,447,000
Total costs and expenses	222,082,000	169,304,000
Loss from operations	(188,788,000)	(147,405,000)
Other income (expense):
Interest income	7,525,000	5,794,000
Other income	43,000	335,000
Total other income	7,568,000	6,129,000
Loss before income taxes	(181,220,000)	(141,276,000)
(Provision) benefit for income taxes	(695,000)	62,000
Net loss	$ (181,915,000)	$ (141,214,000)
Net loss per common share - basic and diluted	$ (1.73)	$ (1.41)
Weighted-average common shares used to compute basic and diluted net loss per common share	105,400	99,979
Comprehensive loss:
Net loss	$ (181,915,000)	$ (141,214,000)
Unrealized gain (loss) on marketable securities, net of tax	360,000	(420,000)
Comprehensive loss	(181,555,000)	(141,634,000)
Product Revenues, Net
Revenues:
Revenues	26,291,000	0
Net Revenues from Collaborators
Revenues:
Revenues	$ 7,003,000	$ 21,899,000

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2017	$ 1,766,431	$ 997	$ 3,947,552	$ (34,433)	$ (2,147,685)
Balance (in shares) at Dec. 31, 2017		99,667
Cumulative effect adjustment from the adoption of new revenue standard	68,210				68,210
Exercise of common stock options, net of tax withholdings	41,890	$ 8	41,882
Exercise of common stock options, net of tax withholdings (in shares)		795
Issuance of common stock under other types of equity plans	567		567
Issuance of common stock under other types of equity plans (in shares)		4
Issuance of common stock under equity plans, net of tax withholdings	(122)		(122)
Issuance of common stock under equity plans, net of tax withholdings (in shares)		2
Stock-based compensation expense related to equity-classified awards	19,463		19,463
Other comprehensive gain (loss), net of tax	(420)			(420)
Net loss	(141,214)				(141,214)
Balance at Mar. 31, 2018	1,754,805	$ 1,005	4,009,342	(34,853)	(2,220,689)
Balance (in shares) at Mar. 31, 2018		100,468
Balance at Dec. 31, 2018	$ 1,301,965	$ 1,011	4,175,139	(33,213)	(2,840,972)
Balance (in shares) at Dec. 31, 2018	101,177	101,177
Exercise of common stock options, net of tax withholdings	$ 11,409	$ 3	11,406
Exercise of common stock options, net of tax withholdings (in shares)		207
Issuance of common stock under other types of equity plans	784		784
Issuance of common stock under other types of equity plans (in shares)		11
Issuance of common stock under equity plans, net of tax withholdings	(58)		(58)
Issuance of common stock under equity plans, net of tax withholdings (in shares)		5
Issuance of common stock, net of offering costs	381,900	$ 50	381,850
Issuance of common stock, net of offering costs (in shares)		5,000
Stock-based compensation expense related to equity-classified awards	32,541		32,541
Other comprehensive gain (loss), net of tax	360			360
Net loss	(181,915)				(181,915)
Balance at Mar. 31, 2019	$ 1,546,986	$ 1,064	$ 4,601,662	$ (32,853)	$ (3,022,887)
Balance (in shares) at Mar. 31, 2019	106,400	106,400

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (181,915)	$ (141,214)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation, amortization and accretion, net	12,210	3,178
Stock-based compensation	32,032	19,584
Charge for 401(k) company stock match	1,089	942
Fair value adjustments on marketable equity securities	(21)
Changes in operating assets and liabilities:
Accounts receivable, net	(15,041)	(16,766)
Proceeds from landlord lease incentive for tenant improvements	12,386
Inventory	(7,134)
Prepaid expenses and other assets	(9,418)	(27,540)
Accounts payable	(17,133)	(9,042)
Accrued expenses and other	(14,731)	(11,767)
Deferred revenue	(834)	26,961
Net cash used in operating activities	(188,510)	(155,664)
Cash flows from investing activities:
Purchases of property, plant and equipment	(44,049)	(21,257)
Purchases of marketable debt securities	(256,996)	(358,433)
Sales and maturities of marketable securities	403,697	244,876
Net cash provided by (used in) investing activities	102,652	(134,814)
Cash flows from financing activities:
Proceeds from exercise of stock options and other types of equity	9,083	42,094
Offering proceeds, net of costs	381,900
Payments for repurchase of common stock for employee tax withholding	(52)	(572)
Net cash provided by financing activities	390,931	41,522
Net increase (decrease) in cash, cash equivalents and restricted cash	305,073	(248,956)
Cash, cash equivalents and restricted cash, beginning of period	422,631	646,832
Cash, cash equivalents and restricted cash, end of period	727,704	397,876
Supplemental disclosure of noncash investing activities:
Capital expenditures included in accounts payable and accrued expenses	$ 13,033	$ 10,437

NATURE OF BUSINESS

NATURE OF BUSINESS	3 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
NATURE OF BUSINESS	1. NATURE OF BUSINESS We commenced operations on June 14, 2002 as a biopharmaceutical company seeking to develop and commercialize novel therapeutics based on RNA interference, or RNAi. We are committed to the advancement of our company strategy of building a multi-product, commercial biopharmaceutical company with a sustainable pipeline of RNAi therapeutics to address the needs of patients who have limited or inadequate treatment options. Since inception, we have focused on discovering, developing and commercializing RNAi therapeutics by establishing and maintaining a strong intellectual property position in the RNAi field, establishing strategic alliances with leading pharmaceutical and life sciences companies, generating revenues through licensing agreements, and ultimately developing and commercializing RNAi therapeutics globally, either independently or with our strategic partners. We have devoted substantially all of our efforts to business planning, research, development, manufacturing and early commercial efforts, acquiring, filing and expanding intellectual property rights, recruiting management and technical staff, and raising capital. In late 2017, we filed a new drug application and a marketing authorisation application seeking regulatory approval of ONPATTRO ® We are subject to risks common to companies in our industry including, but not limited to, uncertainties relating to conducting clinical research and development, the manufacture and supply of products for clinical and commercial use, obtaining and maintaining regulatory approvals and pricing and reimbursement for our products, market acceptance, managing global growth and operating expenses, availability of additional capital, competition, obtaining and enforcing patents, stock price volatility, dependence on collaborative relationships and third-party service providers, dependence on key personnel, potential litigation, product liability claims and government investigations.

BASIS OF PRESENTATION AND PRINC

BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION	2. BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION The accompanying condensed consolidated financial statements of Alnylam Pharmaceuticals, Inc. are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America, or GAAP, applicable to interim periods and, in the opinion of management, include all normal and recurring adjustments that are necessary to state fairly the results of operations for the reported periods. Our condensed consolidated financial statements have also been prepared on a basis substantially consistent with, and should be read in conjunction with, our audited consolidated financial statements for the year ended December 31, 2018, which were included in our Annual Report on Form 10-K that was filed with the Securities and Exchange Commission on February 14, 2019. The year-end condensed consolidated balance sheet data was derived from our audited financial statements but does not include all disclosures required by GAAP. The results of our operations for any interim period are not necessarily indicative of the results of our operations for any other interim period or for a full fiscal year. The accompanying condensed consolidated financial statements reflect the operations of Alnylam and our wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated. Our significant accounting policies are described in Note 2 of the Notes to Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2018. Updates to our significant accounting policies, including the updated lease accounting policy due to the adoption of the new leasing accounting standard, are discussed below and under “Recent Accounting Pronouncements.” Liquidity Based on our current operating plan, we believe that our cash, cash equivalents and marketable debt securities at March 31, 2019, together with the cash we expect to generate from product sales and under our alliances, will be sufficient to enable us to advance our Alnylam 2020 Leases We determine whether a contract is, or contains, a lease at inception. We classify each of our leases as operating or financing considering factors such as the length of the lease term, the present value of the lease payments, the nature of the asset being leased, and the potential for ownership of the asset to transfer during the lease term. Leases with terms greater than one-year are recognized on the condensed consolidated balance sheets as right-of-use assets and lease liabilities and are measured at the present value of the fixed payments due over the expected lease term minus the present value of any incentives, rebates or abatements we expect to receive from the lessor. Options to extend a lease are included in the expected lease term if exercise of the option is deemed reasonably certain. Costs determined to be variable and not based on an index or rate are not included in the measurement of the lease liability and are expensed as incurred. The interest rate implicit in lease contracts is typically not readily determinable. As such, we utilize the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis an amount equal to the lease payments over a similar term and in a similar economic environment. We record expense to recognize fixed lease payments on a straight-line basis over the expected lease term. We have elected the practical expedient not to separate lease and non-lease components for real estate leases. Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board, or FASB, issued a new leasing standard which generally requires lessees to recognize operating and financing lease liabilities and corresponding right-of-use assets on the condensed consolidated balance sheet and to provide enhanced disclosures surrounding the amount, timing and uncertainty of cash flows arising from leasing arrangements. We adopted the new standard on January 1, 2019, using a modified retrospective basis and did not restate comparative periods. In addition, we did not elect the package of practical expedients permitted under the transition guidance that permits companies to carry forward prior conclusions related to (1) whether any expired or existing contracts are, or contain, leases, (2) the lease classification for expired or existing leases, and (3) initial direct costs for existing leases. All our leases have been classified as operating leases under the new leasing standard. We elected to combine lease and non-lease components and to keep leases with an initial term of 12 months or less off the condensed consolidated balance sheets and recognize the associated lease payments in the condensed consolidated statements of comprehensive loss on a straight-line basis over the lease term. Please read Note 8 for additional disclosures related to accounting for leases under this new standard. The adoption of ASC 842 has a material impact on our condensed consolidated balance sheet as the standard requires us to measure and recognize a right of use asset and lease liability. As most leases do not provide an implicit rate, our incremental borrowing rate was determined based on the information available at the date of adoption to measure our lease liability. Costs determined to be variable and not based on an index or rate were not included in the measurement of the lease liability. We recognized approximately $290 million of operating lease liabilities and approximately $230 million of operating lease right-of-use assets on our condensed consolidated balance sheet as of January 1, 2019, which are presented as separate line items on the condensed consolidated balance sheet as of March 31, 2019. Had we not adopted the new leasing standard, we would not have had operating lease right-of-use assets or operating lease liabilities on our condensed consolidated balance sheet as of March 31, 2019. The adoption of the standard did not have a material impact on our condensed consolidated statement of comprehensive income. In March 2017, the FASB issued a new standard that amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. The new standard became effective for us on January 1, 2019. This standard did not have a significant impact on our condensed consolidated financial statements and related disclosures. In August 2018, the FASB issued amendments that eliminate, add and modify certain disclosure requirements on fair value measurements. The amendments become effective for our fiscal year, including interim periods, beginning January 1, 2020. Early adoption of the amendments in full or only the provisions that eliminate or modify the disclosure requirements for fair value measurements is permitted. We are currently evaluating the timing of our adoption and the expected impact that these amendments could have on our disclosures In November 2018, the FASB issued guidance to clarify the interaction between the accounting guidance for collaborative arrangements and revenue from contracts with customers. The amendments become effective for our fiscal year, including interim periods, beginning January 1, 2020. Early adoption, including adoption in any interim period, is permitted. This guidance is required to be applied retrospectively as of the date of our adoption of the new revenue standard on January 1, 2018. We are currently evaluating the timing of our adoption and the expected impact this guidance could have on our condensed consolidated financial statements and related disclosures

PRODUCT REVENUES, NET

PRODUCT REVENUES, NET	3 Months Ended
	Mar. 31, 2019
Revenue From Contract With Customer [Abstract]
PRODUCT REVENUES, NET	3. PRODUCT REVENUES, NET During the three months ended March 31, 2019, we recorded product revenues, net of $26.3 million, which consisted of commercial sales of ONPATTRO primarily in the U.S., along with sales in several European countries. We did not record any product revenues in the three months ended March 31, 2018. As of March 31, 2019 and December 31, 2018, net product revenue related receivables of $22.1 million and $13.1 million, respectively, were included in “Accounts receivable, net.”

COLLABORATION AGREEMENTS

COLLABORATION AGREEMENTS	3 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
COLLABORATION AGREEMENTS	4. COLLABORATION AGREEMENTS The following table summarizes our total condensed Three Months Ended March 31, Description 2019 2018 Sanofi Genzyme $ 4,117 $ 18,853 The Medicines Company 1,745 1,295 Vir Biotechnology 928 1,242 Other 213 509 Total net revenues from collaborators $ 7,003 $ 21,899 The following table presents the balance of our receivables and contract liabilities related to our collaboration agreements at March 31, 2019 and December 31, 2018, in thousands: At March 31, 2019 At December 31, 2018 Receivables included in “Accounts receivable, net” $ 11,723 $ 5,625 Contract liabilities included in “Deferred revenue” 3,026 3,954 During the three months ended March 31, 2019, we recognized the following revenue as a result of the change in the contract liability balances related to our collaboration agreements, in thousands: Revenue recognized in the period from: Three Months Ended March 31, 2019 Amounts included in contract liability at the beginning of the period $ 928 In order to determine revenue recognized in the period from contract liabilities, we first allocate revenue to the individual contract liability balance outstanding at the beginning of the period until the revenue exceeds that balance. If additional consideration is received on those contracts in subsequent periods, we assume all revenue recognized in the reporting period first applies to the beginning contract liability as opposed to a portion applying to the new consideration for the period. The following table provides the research and development expenses incurred by type that are directly attributable to our collaboration agreements by our collaboration partners for the periods indicated, in thousands: Three Months Ended March 31, 2019 2018 Sanofi Genzyme MDCO Vir Sanofi Genzyme MDCO Vir Research and development Clinical trial and manufacturing $ 4,826 $ 1,612 $ 294 $ 10,523 $ 641 $ 554 External services 135 10 236 2,673 — 688 Other 59 50 129 509 — 200 Total research and development expenses $ 5,020 $ 1,672 $ 659 $ 13,705 $ 641 $ 1,442 The research and development expenses incurred for each agreement listed in the table above consist of costs incurred for external development and manufacturing services for which we are reimbursed, licensing payments made to the counterparty to such agreement and costs directly attributable to Sanofi Genzyme transition services. In addition, these expenses include a reasonable estimate of compensation and related costs as billed to our counterparties. As part of our revenue recognition policy adopted on January 1, 2018, the costs in the above table are considered as an input in our determination of transaction price when they relate to consideration received for the delivery of goods or services. For the three months ended March 31, 2019 and 2018, we did not incur material selling, general and administrative expenses related to our collaboration agreements. Sanofi Genzyme Collaboration Collaboration Amendment On April 8, 2019, we and Sanofi Genzyme entered into an amendment to our 2014 Sanofi Genzyme collaboration, which we refer to as the Collaboration Amendment. Under the Collaboration Amendment, we and Sanofi Genzyme agreed to conclude the research and option phase under our collaboration agreement. In connection and simultaneously with entering into the Collaboration Amendment, we and Sanofi Genzyme also entered into the Amended and Restated ALN-AT3 Global License Terms with respect to ALN-AT3 (fitusiran) and certain back-up products, which we refer to as the A&R AT3 License Terms. The A&R AT3 License Terms amend and restate the ALN-AT3 Global License Terms entered into by us and Sanofi Genzyme in January 2018 to modify certain of the business terms. The material collaboration terms for fitusiran, as previously announced, will continue unchanged. In connection with entering into the Collaboration Amendment and the A&R AT3 License Terms, we agreed to advance, at our cost, a selected investigational asset in an undisclosed rare genetic disease through the end of IND-enabling studies. Following completion of such studies, we will transition, at our cost, such asset to Sanofi Genzyme. Thereafter, Sanofi Genzyme will fund all potential future development and commercialization costs for such asset. If this asset is developed and approved, we will be eligible to receive tiered double-digit royalties on global net sales. No changes were made to our Exclusive License Agreement with Sanofi Genzyme, referred to as the Exclusive TTR License, pursuant to which we have global rights for the development and commercialization of ONPATTRO, together with vutrisiran and all back-up products, which remains in full force and effect. Amended and Restated Investor Agreement In connection with the Collaboration Amendment, we and Sanofi Genzyme also entered into an Amended and Restated Investor Agreement, referred to as the A&R Investor Agreement, which amends and restates the Investor Agreement entered into by us and Sanofi Genzyme in February 2014, referred to as the Original Investor Agreement. Pursuant to the A&R Investor Agreement, Sanofi Genzyme is released from the lock-up restrictions under the Original Investor Agreement and is permitted to sell shares of our common stock in transactions approved by us or in fully bought block sale transactions satisfying the conditions set forth in the A&R Investor Agreement. As of January 17, 2019, Sanofi Genzyme owned 10,554,134 shares of our common stock. Under the A&R Investor Agreement, until the earlier of (i) the fifth anniversary of the expiration of the last to expire royalty term or the earlier termination of the collaboration agreement, as amended by the Collaboration Amendment, and (ii) the date after December 31, 2021 on which the beneficial ownership of Sanofi Genzyme and its affiliates no longer represents at least 5% of the outstanding shares of common stock, Sanofi Genzyme and its affiliates will be bound by certain “standstill” provisions, including an agreement not to propose or support a proposal to acquire us. Under the A&R Investor Agreement, Sanofi Genzyme no longer has registration rights or the conditional right to appoint one individual to our board of directors. Sanofi Genzyme continues to be entitled to certain financial information rights until Sanofi Genzyme and its affiliates no longer beneficially own at least 2.5% of our outstanding shares of common stock.

INVENTORY

INVENTORY	3 Months Ended
	Mar. 31, 2019
Inventory Disclosure [Abstract]
INVENTORY	5. INVENTORY The following table presents our inventory at March 31, 2019 and December 31, 2018, in thousands: At March 31, 2019 At December 31, 2018 Raw materials $ 9,179 $ 8,709 Work in progress 22,687 15,262 Finished goods 135 97 Total inventory $ 32,001 $ 24,068

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	6. FAIR VALUE MEASUREMENTS The following tables present information about our assets that are measured at fair value on a recurring basis at March 31, 2019 and December 31, 2018, and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value, in thousands: Description At March 31, 2019 Quoted Prices in Active Markets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: U.S. treasury securities $ 530,231 $ — $ 530,231 $ — Money market funds 98,236 98,236 — — Marketable debt securities: Certificates of deposit 3,000 — 3,000 — Commercial paper 34,394 — 34,394 — Corporate notes 113,253 — 113,253 — U.S. government-sponsored enterprise securities 2,496 — 2,496 — U.S. treasury securities 366,435 — 366,435 — Restricted cash (money market funds) 1,478 1,478 — — Total $ 1,149,523 $ 99,714 $ 1,049,809 $ — Description At December 31, 2018 Quoted Prices in Active Markets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: U.S. treasury securities $ 221,281 $ — $ 221,281 $ — Money market funds 102,445 102,445 — — Marketable debt securities: Certificates of deposit 8,951 — 8,951 — Commercial paper 57,197 — 57,197 — Corporate notes 232,410 — 232,410 — U.S. government-sponsored enterprise securities 39,018 — 39,018 — U.S. treasury securities 325,227 — 325,227 — Marketable equity securities 1,206 1,206 — — Restricted cash (money market funds) 1,477 1,477 — — Total $ 989,212 $ 105,128 $ 884,084 $ — During the three months ended March 31, 2019 and 2018, there were no transfers between Level 1 and Level 2 financial assets. The carrying amounts reflected in our condensed The fair value of our long-term debt at , computed pursuant to a discounted cash flow technique using a market interest rate, was $30.1 million and is considered a Level 3 fair value measurement. The effective interest rate reflects the current market rate.

MARKETABLE DEBT SECURITIES

MARKETABLE DEBT SECURITIES	3 Months Ended
	Mar. 31, 2019
Investments Debt And Equity Securities [Abstract]
MARKETABLE DEBT SECURITIES	7. MARKETABLE DEBT SECURITIES We obtain fair value measurement data for our marketable debt securities from independent pricing services. We perform validation procedures to ensure the reasonableness of this data. This includes meeting with the independent pricing services to understand the methods and data sources used. Additionally, we perform our own review of prices received from the independent pricing services by comparing these prices to other sources and confirming those securities are trading in active markets. We did not record any impairment charges related to our marketable debt securities during the three months ended March 31, 2019 or 2018. The following tables summarize our marketable debt securities at March 31, 2019 and December 31, 2018, in thousands: At March 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Certificates of deposit $ 3,000 $ — $ — $ 3,000 Commercial paper 34,394 — — 34,394 Corporate notes 113,281 1 (29 ) 113,253 U.S. government-sponsored enterprise securities 2,496 — — 2,496 U.S. treasury securities 896,699 25 (58 ) 896,666 Total $ 1,049,870 $ 26 $ (87 ) $ 1,049,809 At December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Certificates of deposit $ 8,951 $ — $ — $ 8,951 Commercial paper 57,197 — — 57,197 Corporate notes 232,695 — (285 ) 232,410 U.S. government-sponsored enterprise securities 39,031 — (13 ) 39,018 U.S. treasury securities 546,631 1 (124 ) 546,508 Total $ 884,505 $ 1 $ (422 ) $ 884,084 The fair values of our marketable debt securities by classification in the condensed consolidated balance sheets were as follows, in thousands: At March 31, 2019 At December 31, 2018 Cash and cash equivalents $ 530,231 $ 221,281 Marketable debt securities 519,578 662,803 Total $ 1,049,809 $ 884,084 We classify our debt security investments based on their contractual maturity dates. The following table summarizes our available-for-sale debt securities by contractual maturity, at March 31, 2019, in thousands: At March 31, 2019 Amortized Fair Value Less than one year $ 1,049,870 $ 1,049,809 Greater than one year but less than two years — — Total $ 1,049,870 $ 1,049,809

LEASES

LEASES	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
LEASES	8. LEASES Overview of Significant Leases We lease three facilities for office and laboratory space in Cambridge, Massachusetts that represent substantially all of our lease obligations. An overview of these significant leases are as follows: 675 West Kendall Street In April 2015, we entered into a non-cancelable real property lease for approximately 295,000 square feet of laboratory and office space located at 675 West Kendall Street, Cambridge, Massachusetts. We intend to move our corporate headquarters and research facility to this location in 2019. The lease commenced on May 1, 2018 and monthly rent payments became due commencing on February 1, 2019 upon substantial completion of the building improvements, and continue for 15 years, with options to renew for two terms of five years each. Exercise of these options was not determined to be reasonably certain and thus was not included in the operating lease liability on the condensed consolidated balance sheet as of March 31, 2019. 300 Third Street We lease approximately 129,000 square feet of office and laboratory space located at 300 Third Street, Cambridge, Massachusetts, which to date has served as our corporate headquarters under a non-cancelable real property lease agreement dated as of September 26, 2003, as amended. The term of the lease expires on January 31, 2034, with an option to extend for two additional five-year terms. Exercise of these options was not determined to be reasonably certain and thus was not included in the operating lease liability on the condensed consolidated balance sheet as of March 31, 2019. 101 Main Street In March and May 2015, we entered into non-cancelable real property lease agreements for approximately 72,000 square feet of office space located on several floors at 101 Main Street, Cambridge, Massachusetts that will expire in March 2024 and June 2021, respectively, each with an option to renew for one five-year term. Exercise of these options was not determined to be reasonably certain and thus was not included in the operating lease liability on the condensed consolidated balance sheet as of March 31, 2019. Other Lease Disclosures Our facility leases described above generally contain customary provisions allowing the landlords to terminate the leases if we fail to remedy a breach of any of our obligations under any such lease within specified time periods, or upon our bankruptcy or insolvency. The below table summarizes our costs included in operating expenses related to leases we have entered into through March 31, 2019: Lease Cost For the Three Months Ended March 31, Operating lease cost $ 9,290 Variable lease cost 5,047 Total $ 14,337 Short-term lease costs were not material for the three months ended March 31, 2019. Net cash paid for the amounts included in the measurement of the operating lease liability on our condensed consolidated balance sheet and included in accrued expenses and other within operating activities in our condensed consolidated statement of cash flow was $7.1 million for the period ending March 31, 2019. The weighted-average remaining lease term and weighted-average discount rate for all leases as of March 31, 2019 was 13.9 years and 8.2%, respectively. Future lease payments for non-cancellable operating leases as of March 31, 2019 and a reconciliation to the carrying amount of the operating lease liability presented in the condensed consolidated balance sheet as of March 31, 2019 is as follows: Year Ending December 31, 2019 (excluding the three months ended March 31, 2019) $ 11,125 2020 34,530 2021 35,776 2022 35,880 2023 35,575 2024 34,674 2025 and thereafter 356,211 Total undiscounted payments due under non-cancellable operating leases 543,771 Less imputed interest (244,494 ) Total $ 299,277 Current operating lease liability $ 16,977 Non-current operating lease liability 282,300 Total $ 299,277 Under the prior lease guidance, minimum payments under our non-cancelable facility leases, as of December 31, 2018, were approximately as follows, in thousands: Year Ending December 31, 2019 $ 32,228 2020 34,826 2021 34,410 2022 34,826 2023 35,270 Thereafter 390,455 Total $ 562,015

CREDIT AGREEMENT

CREDIT AGREEMENT	3 Months Ended
	Mar. 31, 2019
Line Of Credit Facility [Abstract]
CREDIT AGREEMENT	9. CREDIT AGREEMENT On April 29, 2016, we entered into a Credit Agreement, or the Credit Agreement, by and among Alnylam U.S., Inc., as the borrower, us, as a guarantor, and Wells Fargo Bank, National Association, as the lender. The Credit Agreement was entered into in connection with the planned build out of our drug substance manufacturing facility. The Credit Agreement provides for a $30.0 million term loan facility and matures on April 29, 2021. The proceeds of the borrowing under the Credit Agreement are to be used for working capital and general corporate purposes. Interest on borrowings under the Credit Agreement is calculated based on LIBOR plus 0.45 percent, except in the event of default. The borrower may prepay loans under the Credit Agreement at any time, without premium or penalty, subject to certain notice requirements and LIBOR breakage costs. The obligations of the borrower and us under the Credit Agreement are secured by cash collateral in an amount equal to, at any given time, at least 100 percent of the principal amount of all term loans outstanding under such Credit Agreement at such time. At each of March 31, 2019 and December 31, 2018, we recorded $30.0 million of cash collateral in connection with the Credit Agreement as restricted investments on our condensed consolidated balance sheets. The Credit Agreement contains limited representations and warranties and limited affirmative and negative covenants, including quarterly reporting obligations, as well as certain customary events of default.

EQUITY

EQUITY	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
EQUITY	10. EQUITY In January 2019, we sold an aggregate of 5,000,000 shares of our common stock through an underwritten public offering at a price to the public of $77.50 per share. As a result of the offering, we received aggregate net proceeds of $381.9 million after deducting underwriting discounts and commissions and other offering expenses of approximately $5.6 million.

STOCK-BASED COMPENSATION

STOCK-BASED COMPENSATION	3 Months Ended
	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
STOCK-BASED COMPENSATION	11. STOCK-BASED COMPENSATION Stock-based compensation expenses included in operating costs and expenses are as follows, in thousands: Three Months Ended March 31, 2019 2018 Research and development $ 16,125 $ 10,137 Selling, general and administrative 15,907 9,447 Total stock-based compensation $ 32,032 $ 19,584

NET LOSS PER COMMON SHARE

NET LOSS PER COMMON SHARE	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
NET LOSS PER COMMON SHARE	12. NET LOSS PER COMMON SHARE We compute basic net loss per common share by dividing net loss by the weighted-average number of common shares outstanding. We compute diluted net loss per common share by dividing net loss by the weighted-average number of common shares and dilutive potential common share equivalents then outstanding. Potential common shares consist of shares issuable upon the exercise of stock options (the proceeds of which are then assumed to have been used to repurchase outstanding shares using the treasury stock method). Because the inclusion of potential common shares would be anti-dilutive for all periods presented, diluted net loss per common share is the same as basic net loss per common share. The following table sets forth for the periods presented the potential common shares (prior to consideration of the treasury stock method) excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive, in thousands: At March 31, 2019 2018 Options to purchase common stock 13,868 12,515 Unvested restricted common stock 717 157 14,585 12,672

RECONCILIATION OF CASH, CASH EQ

RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH	3 Months Ended
	Mar. 31, 2019
Cash Cash Equivalents And Restricted Cash [Abstract]
RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH	13. RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within our condensed consolidated balance sheets that sum to the total of these amounts shown in the condensed consolidated statements of cash flows, in thousands: At March 31, 2019 2018 Cash and cash equivalents $ 724,959 $ 396,149 Restricted cash included in prepaid expenses and other current assets 439 — Restricted cash included in long-term other assets 2,306 1,727 Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows $ 727,704 $ 397,876

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	3 Months Ended
	Mar. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	14. COMMITMENTS AND CONTINGENCIES Manufacturing Facility In April 2016, we purchased 12 acres of undeveloped land in Norton, Massachusetts. We are constructing a manufacturing facility at this site for drug substance for clinical and commercial use. At March 31, 2019 and December 31, 2018, property, plant and equipment, net, on our condensed consolidated balance sheets reflect $240.7 million and $227.7 million, respectively, of land and associated costs related to the construction of our drug substance manufacturing facility. Litigation From time to time, we are a party to legal proceedings in the course of our business, including the matters described below. The claims and legal proceedings in which we could be involved include challenges to the scope, validity or enforceability of patents relating to our products or product candidates, and challenges by us to the scope, validity or enforceability of the patents held by others. These include claims by third parties that we infringe their patents. The outcome of any such legal proceedings, regardless of the merits, is inherently uncertain. In addition, litigation and related matters are costly and may divert the attention of our management and other resources that would otherwise be engaged in other activities. If we were unable to prevail in any such legal proceedings, our business, results of operations, liquidity and financial condition could be adversely affected Securities Litigation By stipulation of the parties and Order of the Court dated November 20, 2018, the action was transferred to the United States District Court for the District of Massachusetts. Motions for the appointment of lead plaintiff and lead counsel were required to be filed by November 26, 2018. There are three such motions currently pending. We anticipate that an amended complaint will be filed following the Court’s appointment of lead plaintiff and lead counsel. We believe that the allegations contained in the complaint are without merit and intend to defend the case vigorously. We cannot predict at this point the length of time that this action will be ongoing or the liability, if any, which may arise therefrom. Dicerna Litigation On June 10, 2015, we filed a trade secret misappropriation lawsuit against Dicerna Pharmaceuticals, Inc., or Dicerna, in the Superior Court of Middlesex County, Massachusetts seeking to stop misappropriation by Dicerna of our confidential, proprietary and trade secret information related to the RNAi assets we purchased from Merck Sharp & Dohme Corp., including certain N-acetylgalactosamine, or GalNAc, conjugate technology. In addition to permanent injunctive relief, we were also seeking monetary damages from Dicerna. On April 18, 2018, we and Dicerna entered into a settlement agreement resolving all ongoing litigation between the companies. Under the terms of the settlement agreement, Dicerna was required to pay us an aggregate of $25.0 million, including an upfront cash payment of $2.0 million and 983,208 shares of Dicerna common stock, valued at $10.0 million, that were received in the second quarter of 2018, and an additional $13.0 million over the next four years, the timing of which was dependent upon revenue Dicerna received pursuant to future partnerships and collaborations related to GalNAc-conjugated RNAi research and development. As a result of Dicerna collaborations entered into subsequent to the settlement agreement, we were due the remaining $13.0 million as of December 31, 2018, of which $2.5 million was received in December 2018 and $10.5 million was received in January 2019.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	3 Months Ended
	Mar. 31, 2019
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	15. SUBSEQUENT EVENTS On April 8, 2019, we entered into a global, strategic collaboration with Regeneron Pharmaceuticals, Inc., or Regeneron, to discover, develop and commercialize RNAi therapeutics for a broad range of diseases by addressing disease targets expressed in the eye and central nervous system, or CNS, in addition to a select number of targets expressed in the liver. In connection with the collaboration, Regeneron has agreed to make a $400.0 million upfront payment to us and we are eligible to receive up to an additional $200.0 million in milestone payments upon achievement of certain criteria during early clinical development for the eye and CNS programs. The companies plan to advance programs directed to 30 targets during the initial five-year discovery period, which may be extended under certain circumstances for an additional two years. Regeneron has the option to extend the initial research term upon payment of a research term extension fee. For each program, Regeneron will provide us with $2.5 million in funding at program initiation and an additional $2.5 million at lead candidate identification, with the potential for approximately $30.0 million in annual discovery funding to us as the collaboration reaches steady state. In conjunction with the collaboration agreement, Regeneron entered into a stock purchase agreement with us whereby Regeneron has agreed to purchase $400.0 million of our common stock at a price per share of $90.00 (4.44 million shares), based on the volume-weighted average price over the fifteen-trading-day period preceding execution of the stock purchase agreement. The collaboration will become effective upon closing of the equity transaction, subject to clearance under the Hart-Scott Rodino Antitrust Improvements Act of 1976, as amended, and other customary closing conditions, which we expect to occur in the second quarter of 2019.

BASIS OF PRESENTATION AND PRI_2

BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION (Policies)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION The accompanying condensed consolidated financial statements of Alnylam Pharmaceuticals, Inc. are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America, or GAAP, applicable to interim periods and, in the opinion of management, include all normal and recurring adjustments that are necessary to state fairly the results of operations for the reported periods. Our condensed consolidated financial statements have also been prepared on a basis substantially consistent with, and should be read in conjunction with, our audited consolidated financial statements for the year ended December 31, 2018, which were included in our Annual Report on Form 10-K that was filed with the Securities and Exchange Commission on February 14, 2019. The year-end condensed consolidated balance sheet data was derived from our audited financial statements but does not include all disclosures required by GAAP. The results of our operations for any interim period are not necessarily indicative of the results of our operations for any other interim period or for a full fiscal year. The accompanying condensed consolidated financial statements reflect the operations of Alnylam and our wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated. Our significant accounting policies are described in Note 2 of the Notes to Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2018. Updates to our significant accounting policies, including the updated lease accounting policy due to the adoption of the new leasing accounting standard, are discussed below and under “Recent Accounting Pronouncements.”
Liquidity	Liquidity Based on our current operating plan, we believe that our cash, cash equivalents and marketable debt securities at March 31, 2019, together with the cash we expect to generate from product sales and under our alliances, will be sufficient to enable us to advance our Alnylam 2020
Leases	Leases We determine whether a contract is, or contains, a lease at inception. We classify each of our leases as operating or financing considering factors such as the length of the lease term, the present value of the lease payments, the nature of the asset being leased, and the potential for ownership of the asset to transfer during the lease term. Leases with terms greater than one-year are recognized on the condensed consolidated balance sheets as right-of-use assets and lease liabilities and are measured at the present value of the fixed payments due over the expected lease term minus the present value of any incentives, rebates or abatements we expect to receive from the lessor. Options to extend a lease are included in the expected lease term if exercise of the option is deemed reasonably certain. Costs determined to be variable and not based on an index or rate are not included in the measurement of the lease liability and are expensed as incurred. The interest rate implicit in lease contracts is typically not readily determinable. As such, we utilize the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis an amount equal to the lease payments over a similar term and in a similar economic environment. We record expense to recognize fixed lease payments on a straight-line basis over the expected lease term. We have elected the practical expedient not to separate lease and non-lease components for real estate leases.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board, or FASB, issued a new leasing standard which generally requires lessees to recognize operating and financing lease liabilities and corresponding right-of-use assets on the condensed consolidated balance sheet and to provide enhanced disclosures surrounding the amount, timing and uncertainty of cash flows arising from leasing arrangements. We adopted the new standard on January 1, 2019, using a modified retrospective basis and did not restate comparative periods. In addition, we did not elect the package of practical expedients permitted under the transition guidance that permits companies to carry forward prior conclusions related to (1) whether any expired or existing contracts are, or contain, leases, (2) the lease classification for expired or existing leases, and (3) initial direct costs for existing leases. All our leases have been classified as operating leases under the new leasing standard. We elected to combine lease and non-lease components and to keep leases with an initial term of 12 months or less off the condensed consolidated balance sheets and recognize the associated lease payments in the condensed consolidated statements of comprehensive loss on a straight-line basis over the lease term. Please read Note 8 for additional disclosures related to accounting for leases under this new standard. The adoption of ASC 842 has a material impact on our condensed consolidated balance sheet as the standard requires us to measure and recognize a right of use asset and lease liability. As most leases do not provide an implicit rate, our incremental borrowing rate was determined based on the information available at the date of adoption to measure our lease liability. Costs determined to be variable and not based on an index or rate were not included in the measurement of the lease liability. We recognized approximately $290 million of operating lease liabilities and approximately $230 million of operating lease right-of-use assets on our condensed consolidated balance sheet as of January 1, 2019, which are presented as separate line items on the condensed consolidated balance sheet as of March 31, 2019. Had we not adopted the new leasing standard, we would not have had operating lease right-of-use assets or operating lease liabilities on our condensed consolidated balance sheet as of March 31, 2019. The adoption of the standard did not have a material impact on our condensed consolidated statement of comprehensive income. In March 2017, the FASB issued a new standard that amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. The new standard became effective for us on January 1, 2019. This standard did not have a significant impact on our condensed consolidated financial statements and related disclosures. In August 2018, the FASB issued amendments that eliminate, add and modify certain disclosure requirements on fair value measurements. The amendments become effective for our fiscal year, including interim periods, beginning January 1, 2020. Early adoption of the amendments in full or only the provisions that eliminate or modify the disclosure requirements for fair value measurements is permitted. We are currently evaluating the timing of our adoption and the expected impact that these amendments could have on our disclosures In November 2018, the FASB issued guidance to clarify the interaction between the accounting guidance for collaborative arrangements and revenue from contracts with customers. The amendments become effective for our fiscal year, including interim periods, beginning January 1, 2020. Early adoption, including adoption in any interim period, is permitted. This guidance is required to be applied retrospectively as of the date of our adoption of the new revenue standard on January 1, 2018. We are currently evaluating the timing of our adoption and the expected impact this guidance could have on our condensed consolidated financial statements and related disclosures

COLLABORATION AGREEMENTS (Table

COLLABORATION AGREEMENTS (Tables)	3 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Revenue from Collaborators	The following table summarizes our total condensed Three Months Ended March 31, Description 2019 2018 Sanofi Genzyme $ 4,117 $ 18,853 The Medicines Company 1,745 1,295 Vir Biotechnology 928 1,242 Other 213 509 Total net revenues from collaborators $ 7,003 $ 21,899
Balance and Change in Receivables and Contract Liabilities Related to Collaboration Agreements	The following table presents the balance of our receivables and contract liabilities related to our collaboration agreements at March 31, 2019 and December 31, 2018, in thousands: At March 31, 2019 At December 31, 2018 Receivables included in “Accounts receivable, net” $ 11,723 $ 5,625 Contract liabilities included in “Deferred revenue” 3,026 3,954 During the three months ended March 31, 2019, we recognized the following revenue as a result of the change in the contract liability balances related to our collaboration agreements, in thousands: Revenue recognized in the period from: Three Months Ended March 31, 2019 Amounts included in contract liability at the beginning of the period $ 928
Schedule of Research and Development Expenses Incurred by Type that are Directly Attributable to Collaboration Agreements	The following table provides the research and development expenses incurred by type that are directly attributable to our collaboration agreements by our collaboration partners for the periods indicated, in thousands: Three Months Ended March 31, 2019 2018 Sanofi Genzyme MDCO Vir Sanofi Genzyme MDCO Vir Research and development Clinical trial and manufacturing $ 4,826 $ 1,612 $ 294 $ 10,523 $ 641 $ 554 External services 135 10 236 2,673 — 688 Other 59 50 129 509 — 200 Total research and development expenses $ 5,020 $ 1,672 $ 659 $ 13,705 $ 641 $ 1,442

INVENTORY (Tables)

INVENTORY (Tables)	3 Months Ended
	Mar. 31, 2019
Inventory Disclosure [Abstract]
Schedule of Inventory	The following table presents our inventory at March 31, 2019 and December 31, 2018, in thousands: At March 31, 2019 At December 31, 2018 Raw materials $ 9,179 $ 8,709 Work in progress 22,687 15,262 Finished goods 135 97 Total inventory $ 32,001 $ 24,068

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value of Assets Measured on a Recurring Basis	The following tables present information about our assets that are measured at fair value on a recurring basis at March 31, 2019 and December 31, 2018, and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value, in thousands: Description At March 31, 2019 Quoted Prices in Active Markets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: U.S. treasury securities $ 530,231 $ — $ 530,231 $ — Money market funds 98,236 98,236 — — Marketable debt securities: Certificates of deposit 3,000 — 3,000 — Commercial paper 34,394 — 34,394 — Corporate notes 113,253 — 113,253 — U.S. government-sponsored enterprise securities 2,496 — 2,496 — U.S. treasury securities 366,435 — 366,435 — Restricted cash (money market funds) 1,478 1,478 — — Total $ 1,149,523 $ 99,714 $ 1,049,809 $ — Description At December 31, 2018 Quoted Prices in Active Markets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: U.S. treasury securities $ 221,281 $ — $ 221,281 $ — Money market funds 102,445 102,445 — — Marketable debt securities: Certificates of deposit 8,951 — 8,951 — Commercial paper 57,197 — 57,197 — Corporate notes 232,410 — 232,410 — U.S. government-sponsored enterprise securities 39,018 — 39,018 — U.S. treasury securities 325,227 — 325,227 — Marketable equity securities 1,206 1,206 — — Restricted cash (money market funds) 1,477 1,477 — — Total $ 989,212 $ 105,128 $ 884,084 $ —

MARKETABLE DEBT SECURITIES (Tab

MARKETABLE DEBT SECURITIES (Tables)	3 Months Ended
	Mar. 31, 2019
Investments Debt And Equity Securities [Abstract]
Summary of Company's Marketable Debt Securities	The following tables summarize our marketable debt securities at March 31, 2019 and December 31, 2018, in thousands: At March 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Certificates of deposit $ 3,000 $ — $ — $ 3,000 Commercial paper 34,394 — — 34,394 Corporate notes 113,281 1 (29 ) 113,253 U.S. government-sponsored enterprise securities 2,496 — — 2,496 U.S. treasury securities 896,699 25 (58 ) 896,666 Total $ 1,049,870 $ 26 $ (87 ) $ 1,049,809 At December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Certificates of deposit $ 8,951 $ — $ — $ 8,951 Commercial paper 57,197 — — 57,197 Corporate notes 232,695 — (285 ) 232,410 U.S. government-sponsored enterprise securities 39,031 — (13 ) 39,018 U.S. treasury securities 546,631 1 (124 ) 546,508 Total $ 884,505 $ 1 $ (422 ) $ 884,084
Summary of Fair Value of Marketable Debt Securities	The fair values of our marketable debt securities by classification in the condensed consolidated balance sheets were as follows, in thousands: At March 31, 2019 At December 31, 2018 Cash and cash equivalents $ 530,231 $ 221,281 Marketable debt securities 519,578 662,803 Total $ 1,049,809 $ 884,084
Summary of Available-For-Sale Debt Securities by Contractual Maturity	The following table summarizes our available-for-sale debt securities by contractual maturity, at March 31, 2019, in thousands: At March 31, 2019 Amortized Fair Value Less than one year $ 1,049,870 $ 1,049,809 Greater than one year but less than two years — — Total $ 1,049,870 $ 1,049,809

LEASES (Tables)

LEASES (Tables)	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Summary of Costs Included in Operating Expenses Related to Leases	The below table summarizes our costs included in operating expenses related to leases we have entered into through March 31, 2019: Lease Cost For the Three Months Ended March 31, Operating lease cost $ 9,290 Variable lease cost 5,047 Total $ 14,337
Summary of Future Lease Payments for Non-cancellable Operating Leases and Reconciliation to Carrying Amount of Operating Lease Liability	Future lease payments for non-cancellable operating leases as of March 31, 2019 and a reconciliation to the carrying amount of the operating lease liability presented in the condensed consolidated balance sheet as of March 31, 2019 is as follows: Year Ending December 31, 2019 (excluding the three months ended March 31, 2019) $ 11,125 2020 34,530 2021 35,776 2022 35,880 2023 35,575 2024 34,674 2025 and thereafter 356,211 Total undiscounted payments due under non-cancellable operating leases 543,771 Less imputed interest (244,494 ) Total $ 299,277 Current operating lease liability $ 16,977 Non-current operating lease liability 282,300 Total $ 299,277
Future Minimum Payments Under Non-cancelable Leases	Under the prior lease guidance, minimum payments under our non-cancelable facility leases, as of December 31, 2018, were approximately as follows, in thousands: Year Ending December 31, 2019 $ 32,228 2020 34,826 2021 34,410 2022 34,826 2023 35,270 Thereafter 390,455 Total $ 562,015

STOCK-BASED COMPENSATION (Table

STOCK-BASED COMPENSATION (Tables)	3 Months Ended
	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock Based Compensation	Stock-based compensation expenses included in operating costs and expenses are as follows, in thousands: Three Months Ended March 31, 2019 2018 Research and development $ 16,125 $ 10,137 Selling, general and administrative 15,907 9,447 Total stock-based compensation $ 32,032 $ 19,584

NET LOSS PER COMMON SHARE (Tabl

NET LOSS PER COMMON SHARE (Tables)	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Common Shares Excluded from the Calculation of Net Loss Per Common Share	The following table sets forth for the periods presented the potential common shares (prior to consideration of the treasury stock method) excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive, in thousands: At March 31, 2019 2018 Options to purchase common stock 13,868 12,515 Unvested restricted common stock 717 157 14,585 12,672

RECONCILIATION OF CASH, CASH _2

RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH (Tables)	3 Months Ended
	Mar. 31, 2019
Cash Cash Equivalents And Restricted Cash [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents And Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within our condensed consolidated balance sheets that sum to the total of these amounts shown in the condensed consolidated statements of cash flows, in thousands: At March 31, 2019 2018 Cash and cash equivalents $ 724,959 $ 396,149 Restricted cash included in prepaid expenses and other current assets 439 — Restricted cash included in long-term other assets 2,306 1,727 Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows $ 727,704 $ 397,876

Basis of Presentation and Pri_3

Basis of Presentation and Principles of Consolidation - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Accounting Policies [Abstract]
Operating lease liabilities	$ 299,277	$ 290,000
Operating lease right-of-use assets	$ 226,412	$ 230,000

Product Revenues, Net - Additio

Product Revenues, Net - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Disaggregation Of Revenue [Line Items]
Revenues	$ 33,294,000	$ 21,899,000
Accounts receivable, net	33,801,000		$ 18,760,000
Product Revenues, Net
Disaggregation Of Revenue [Line Items]
Revenues	26,291,000	$ 0
Accounts receivable, net	$ 22,100,000		$ 13,100,000

Revenue from Collaborators (Det

Revenue from Collaborators (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	$ 33,294	$ 21,899
Net Revenues from Collaborators
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	7,003	21,899
Net Revenues from Collaborators | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	4,117	18,853
Net Revenues from Collaborators | The Medicines Company
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	1,745	1,295
Net Revenues from Collaborators | Vir Biotechnology
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	928	1,242
Net Revenues from Collaborators | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	$ 213	$ 509

Balance of Receivables and Cont

Balance of Receivables and Contract Liabilities Related to Collaboration Agreements (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Receivables included in “Accounts receivable, net”	$ 11,723	$ 5,625
Contract liabilities included in “Deferred revenue”	$ 3,026	$ 3,954

Change in Contract Liability Ba

Change in Contract Liability Balance Related to Collaboration Agreements (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Revenue recognized in the period from:
Amounts included in contract liability at the beginning of the period	$ 928

Schedule of Research and Develo

Schedule of Research and Development Expenses Incurred by Type that are Directly Attributable to Collaboration Agreements (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 129,127	$ 96,857
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	5,020	13,705
Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	1,672	641
Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	659	1,442
Clinical trial and manufacturing | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	4,826	10,523
Clinical trial and manufacturing | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	1,612	641
Clinical trial and manufacturing | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	294	554
External services | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	135	2,673
External services | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	10
External services | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	236	688
Other | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	59	509
Other | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	50
Other | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 129	$ 200

Collaboration Agreements - Addi

Collaboration Agreements - Additional Information (Detail) - shares	3 Months Ended
	Mar. 31, 2019	Jan. 17, 2019	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common Stock Shares owned	106,400,000		101,177,000
Sanofi Genzyme | Collaboration Amendment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Investor agreement description	until the earlier of (i) the fifth anniversary of the expiration of the last to expire royalty term or the earlier termination of the collaboration agreement, as amended by the Collaboration Amendment, and (ii) the date after December 31, 2021 on which the beneficial ownership of Sanofi Genzyme and its affiliates no longer represents at least 5% of the outstanding shares of common stock, Sanofi Genzyme and its affiliates will be bound by certain “standstill” provisions, including an agreement not to propose or support a proposal to acquire us.
Sanofi Genzyme | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage ownership interest owned by noncontrolling owners	2.50%
Sanofi Genzyme | Minimum | Collaboration Amendment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage ownership interest owned by noncontrolling owners	5.00%
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common Stock Shares owned		10,554,134

Schedule of Inventory (Detail)

Schedule of Inventory (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Inventory Disclosure [Abstract]
Raw materials	$ 9,179	$ 8,709
Work in progress	22,687	15,262
Finished goods	135	97
Total inventory	$ 32,001	$ 24,068

Fair Value of Assets Measured o

Fair Value of Assets Measured on a Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 530,231	$ 221,281
Available for sale debt securities, Fair value disclosure	1,049,809	884,084
Marketable equity securities		1,206
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities		1,206
Total	1,149,523	989,212
Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities		1,206
Total	99,714	105,128
Recurring | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,049,809	884,084
Recurring | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	530,231	221,281
Available for sale debt securities, Fair value disclosure	366,435	325,227
Recurring | U.S. treasury securities | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	530,231	221,281
Available for sale debt securities, Fair value disclosure	366,435	325,227
Recurring | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	98,236	102,445
Restricted cash	1,478	1,477
Recurring | Money Market Funds | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	98,236	102,445
Restricted cash	1,478	1,477
Recurring | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	3,000	8,951
Recurring | Certificate of deposit | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	3,000	8,951
Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	34,394	57,197
Recurring | Commercial paper | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	34,394	57,197
Recurring | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	113,253	232,410
Recurring | Corporate notes | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	113,253	232,410
Recurring | U.S. government-sponsored enterprise securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	2,496	39,018
Recurring | U.S. government-sponsored enterprise securities | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	$ 2,496	$ 39,018

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long-term debt	$ 30.1	$ 30.1

Marketable Debt Securities - Ad

Marketable Debt Securities - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Marketable Securities [Abstract]
Impairment charges of marketable debt securities	$ 0	$ 0

Summary of Company's Marketable

Summary of Company's Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,049,870	$ 884,505
Gross Unrealized Gains	26	1
Gross Unrealized Losses	(87)	(422)
Fair Value	1,049,809	884,084
Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,000	8,951
Fair Value	3,000	8,951
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	34,394	57,197
Fair Value	34,394	57,197
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113,281	232,695
Gross Unrealized Gains	1
Gross Unrealized Losses	(29)	(285)
Fair Value	113,253	232,410
U.S. government-sponsored enterprise securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,496	39,031
Gross Unrealized Losses		(13)
Fair Value	2,496	39,018
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	896,699	546,631
Gross Unrealized Gains	25	1
Gross Unrealized Losses	(58)	(124)
Fair Value	$ 896,666	$ 546,508

Summary of Fair Value of Market

Summary of Fair Value of Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Cash equivalents	$ 530,231	$ 221,281
Marketable debt securities	519,578	662,803
Total	$ 1,049,809	$ 884,084

Summary of Available-For-Sale D

Summary of Available-For-Sale Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost
Less than one year	$ 1,049,870
Amortized Cost	1,049,870	$ 884,505
Fair Value
Less than one year	1,049,809
Total	$ 1,049,809	$ 884,084

Leases - Additional Information

Leases - Additional Information (Detail) $ in Millions	Sep. 26, 2003ft²RenewalOption	May 31, 2015ft²RenewalOption	Apr. 30, 2015ft²RenewalOption	Mar. 31, 2015	Mar. 31, 2019USD ($)
Operating Leased Assets [Line Items]
Net cash paid included in operating activities in cash flow | $					$ 7.1
Operating lease, weighted-average remaining lease term					13 years 10 months 24 days
Operating lease, weighted-average discount rate					8.20%
BMR-675 West Kendall Lease
Operating Leased Assets [Line Items]
Area of office and laboratory space held under lease | ft²			295,000
Lease term			15 years
Number of lease extension options | RenewalOption			2
Lease renewal options period			5 years
Lessee, operating lease, option to extend					with options to renew for two terms of five years each.
Lessee, operating lease, existence of option to terminate [true false]					true
Third Street Lease
Operating Leased Assets [Line Items]
Area of office and laboratory space held under lease | ft²	129,000
Number of lease extension options | RenewalOption	2
Lease renewal options period	5 years
Lessee, operating lease, option to extend					with an option to extend for two additional five-year terms
Lessee, operating lease, existence of option to terminate [true false]					true
Lease extended expiration date	Jan. 31, 2034
101 Main Street Lease
Operating Leased Assets [Line Items]
Area of office and laboratory space held under lease | ft²		72,000
Number of lease extension options | RenewalOption		1
Lessee, operating lease, option to extend					each with an option to renew for one five-year term.
Lessee, operating lease, existence of option to terminate [true false]					true
Lease extended expiration date		Jun. 30, 2021		Mar. 31, 2024
Operating lease renewal options period		5 years

Leases - Summary of Costs Inclu

Leases - Summary of Costs Included in Operating Expenses Related to Leases (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Lessee Disclosure [Abstract]
Operating lease cost	$ 9,290
Variable lease cost	5,047
Total	$ 14,337

Leases - Summary of Future Leas

Leases - Summary of Future Lease Payments for Non-cancellable Operating Leases and Reconciliation to Carrying Amount of Operating Lease Liability (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Lessee Disclosure [Abstract]
2019 (excluding the three months ended March 31, 2019)	$ 11,125
2020	34,530
2021	35,776
2022	35,880
2023	35,575
2024	34,674
2025 and thereafter	356,211
Total undiscounted payments due under non-cancellable operating leases	543,771
Less imputed interest	(244,494)
Total	299,277	$ 290,000
Operating lease liability	16,977
Operating lease liability, net of current portion	$ 282,300

Leases - Future Minimum Payment

Leases - Future Minimum Payments Under Non-cancelable Leases (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Lessee Disclosure [Abstract]
2019	$ 32,228
2020	34,826
2021	34,410
2022	34,826
2023	35,270
Thereafter	390,455
Total	$ 562,015

Credit Agreement - Additional I

Credit Agreement - Additional Information (Detail) - USD ($)	Apr. 29, 2016	Mar. 31, 2019	Dec. 31, 2018
Line Of Credit Facility [Line Items]
Restricted investments		$ 44,825,000	$ 44,825,000
Wells Fargo Bank, National Association
Line Of Credit Facility [Line Items]
Cash collateral required for principal amount outstanding, percentage	100.00%
Restricted investments		$ 30,000,000	$ 30,000,000
Wells Fargo Bank, National Association | Term Loan Facility
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 30,000,000
Line of credit facility, expiration date	Apr. 29, 2021
Wells Fargo Bank, National Association | Term Loan Facility | LIBOR
Line Of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	0.45%

Equity - Additional Information

Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2019	Mar. 31, 2019
Equity [Abstract]
Issuance of common stock, net of offering costs (in shares)	5,000,000
Underwritten public offering amount per share	$ 77.50
Issuance of common stock, net of offering costs	$ 381,900	$ 381,900
Underwriting discounts and commissions and other offering expenses	$ 5,600

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Share-Based Compensation Expenses Included Operating Costs and Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 32,032	$ 19,584
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	16,125	10,137
Selling, General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 15,907	$ 9,447

Potential Common Shares Exclude

Potential Common Shares Excluded from Calculation of Net Loss Per Common Share (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	14,585	12,672
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	13,868	12,515
Unvested restricted common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	717	157

Reconciliation of Cash, Cash _3

Reconciliation of Cash, Cash Equivalents and Restricted Cash - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash Cash Equivalents And Restricted Cash [Abstract]
Cash and cash equivalents	$ 724,959	$ 420,146	$ 396,149
Restricted cash included in prepaid expenses and other current assets	$ 439
Restricted Cash and Cash Equivalents, Current, Asset, Statement of Financial Position [Extensible List]	us-gaap:PrepaidExpenseAndOtherAssetsCurrent		us-gaap:PrepaidExpenseAndOtherAssetsCurrent
Restricted cash included in long-term other assets	$ 2,306		$ 1,727
Restricted Cash and Cash Equivalents, Noncurrent, Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent		us-gaap:OtherAssetsNoncurrent
Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows	$ 727,704	$ 422,631	$ 397,876	$ 646,832

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Thousands	Apr. 18, 2018USD ($)shares	Jan. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2018USD ($)	Mar. 31, 2019USD ($)	Apr. 30, 2016a
Commitments And Contingencies [Line Items]
Property, plant and equipment, net			$ 320,658	$ 320,658	$ 341,712
Litigation settlement, common stock issuable from other party | shares	983,208
Dicerna Pharmaceuticals, Inc
Commitments And Contingencies [Line Items]
Litigation settlement, settlement agreement date	April 18, 2018
Litigation settlement amount	$ 25,000	$ 10,500	2,500	13,000
Litigation settlement, common stock value	10,000
Additional payment due from litigation settlement	$ 13,000
Litigation settlement payment period	4 years
Up Front Payment | Dicerna Pharmaceuticals, Inc
Commitments And Contingencies [Line Items]
Litigation settlement amount	$ 2,000
Norton, Massachusetts
Commitments And Contingencies [Line Items]
Undeveloped land acquired | a						12
Norton, Massachusetts | Land and costs related to the construction of manufacturing facility
Commitments And Contingencies [Line Items]
Property, plant and equipment, net			$ 227,700	$ 227,700	$ 240,700

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail)	Apr. 08, 2019USD ($)Program$ / sharesshares	Jan. 31, 2019USD ($)$ / sharesshares	Mar. 31, 2019USD ($)
Subsequent Event [Line Items]
Issuance of common stock, net of offering costs		$ 381,900,000	$ 381,900,000
Common stock at a price per share | $ / shares		$ 77.50
Issuance of common stock, net of offering costs (in shares) | shares		5,000,000
Subsequent Event | Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc.
Subsequent Event [Line Items]
Upfront fee received	$ 400,000,000
Maximum additional milestone payments to be receive upon achievement of certain criteria	$ 200,000,000
Number of targeted programs | Program	30
Discovery period of programs development	5 years
Extended additional discovery period of programs development	2 years
Issuance of common stock, net of offering costs	$ 400,000,000
Common stock at a price per share | $ / shares	$ 90
Issuance of common stock, net of offering costs (in shares) | shares	4,440,000
Number of trading period	15 days
Subsequent Event | Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Program Initiation
Subsequent Event [Line Items]
Potential proceeds from collaboration arrangement	$ 2,500,000
Subsequent Event | Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Lead Candidate Identification
Subsequent Event [Line Items]
Potential proceeds from collaboration arrangement	2,500,000
Subsequent Event | Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Steady State
Subsequent Event [Line Items]
Potential proceeds from collaboration arrangement	$ 30,000,000